void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    vec2 uv = fragCoord.xy / iResolution.xy;
    float maskAlpha = texture(iChannel0, uv).a;
    
    fragColor = mix(vec4(0.0, 0.0, 0.0, 0.0), vec4(1.0, 1.0, 1.0, 1.0), maskAlpha);
}
